1. Field of the Invention
The present invention relates to liquid immersion photolithography, and more particularly, to a method and system for confining liquid flow in an immersion photolithographic system.
2. Description of the Related Art
Optical lithography, using lens systems and catadioptric systems, is used extensively in the semiconductor manufacturing industry for the printing of circuit patterns. To date, the gap between a final lens element and a semiconductor wafer surface has been filled with gas, usually air or nitrogen. This gaseous gap works well particularly when the wafer is scanned under the optics during exposure and there is relative movement between the wafer and the lens system during the image transfer.
The practical limits of optical lithography assume that the medium through which imaging is occurring is air. This practical limit is defined by the equation       Λ    =          λ              4        ·        n        ·        NA              ,
where xcex is the wavelength of incident light, NA is numerical aperture of the projection optical system, and n is the index of refraction of the medium (where 4 is used instead of 2 due to the use of off axis illumination). The gas interface between the final lens element and the wafer surface limits the maximum resolution of the optical system to a numerical aperture of  less than 1.0. If the gas space between the final lens element and the wafer surface can be filled with a refractive material, such as oil or water, then the numerical aperture, and hence the resolution capability, of the system can be significantly increased, corresponding to the index of refraction n.
Thus, by introducing a liquid between a last lens element of the projection optical system and a wafer being imaged, the refractive index changes, thereby enabling enhanced resolution with a lower effective wavelength of the light source. Immersion lithography effectively lowers a 157 nm light source to a 115 nm wavelength (for example, for n=1.365), enabling the printing of critical layers with the same photolithographic tools that the industry is accustomed to using today.
Similarly, immersion lithography can push 193 nm lithography down to, for example, 145 nm (for n=1.33). 435 nm, 405 nm, 365 nm, 248 nm, 193 nm and 157 nm tools can all be used to achieve effectively better resolution and xe2x80x9cextendxe2x80x9d the usable wavelengths. Also, large amounts of CaF2, hard pellicles, a nitrogen purge, etc.xe2x80x94can be avoided. Also, depth of focus can be increased by the use of liquid immersion, which may be useful, for example, for LCD panel manufacturing.
However, despite the promise of immersion photolithography, a number of problems remain, which have so far precluded commercialization of immersion photolithographic systems. One problem of existing immersion photolithographic systems involves the difficulties of confining the liquid that is used in an interface between the projection optical system and the wafer being exposed. In conventional systems, liquid is injected between the projection optical system and the wafer. Fairly complex systems have been proposed in order to maintain the confinement of the liquid.
An additional problem exists where the scanning motion of the wafer is such that the wafer is moved away from the exposure area, resulting in a spilling of the liquid. Such spillage is also a problem even when the wafer is present under the projection optical system due to the inherent viscosity properties of the liquid.
Accordingly, what is needed is a simple system and method for confining the liquid between the projection optical system and the wafer.
The present invention is directed to an immersion photolithography system and method using an inverted wafer-projection optics interface that substantially obviates one or more of the problems and disadvantages of the related art.
There is provided a liquid immersion photolithography system including an exposure system that exposes a substrate with electromagnetic radiation, and also includes a projection optical system that focuses the electromagnetic radiation on the substrate. A liquid supply system provides a liquid between the projection optical system and the substrate. The projection optical system is positioned below the substrate.
In another aspect there is provided a liquid immersion photolithography system that includes an exposure system that exposes a substrate with electromagnetic radiation, and also includes a projection optical system that focuses the electromagnetic radiation on the substrate A means for providing a liquid is between the projection optical system and the substrate. The projection optical system is positioned below the substrate. A meniscus is formed between the projection optical system and the wafer.
In another aspect there is provided a method of exposing a substrate including positioning a projection optical system below the substrate, projecting electromagnetic radiation onto the substrate using a projection optical system, and delivering a liquid between the projection optical system and the substrate.
Additional features and advantages of the invention will be set forth in the description that follows. Yet further features and advantages will be apparent to a person skilled in the art based on the description set forth herein or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.